If only和Only if有何分別?

Very hard to explain....

If only = 如果係咁就好
1. If only I had a million dollars, I would buy an Aston Martin right away.
2. If only i had known that she's a man, I wouldn't make that mistake.


Only if = 合乎呢個條件就...
1. I can buy an Aston Martin only if i have more than 2 million dollars in my account.
2. Only if you've done your homework can you then watch TV.